Why do mid-latitude cyclones spin counterclockwise?

low pressure rotates counter-clockwise in the northern hemisphere----and clockwise in the Southern Hemisphere.

This is true for large areas of low pressure systems. Sometimes in rare cases, very small, microscale areas of low pressure (such as a tornado) can go the opposite way, but any low pressure that's more than a couple miles wide will most definitely follow the rules stated above. Even tornadoes typically follow the rule and it's a very uncommon event for them to rotate backwards.